To prevent product piracy in the compact disc (="CD") business, several proposals are known, predominantly to assure the customer that he is buying an authorized CD in the original package. A hologram label used as a seal prevents the breaking of the compact disc cover. Nowadays not only the CD itself is counterfeited but also the original package and the seal.
The GB-A 2 290 526 proposes to attach a hologram label to the information label on the non-readable side of the CD. The information label carries printed information and may incorporate a hologram to enhance security or for artistic effects. The information label is laminated to the non-readable side of the CD. Part of the printing comprises machine-readable information, such as a bar code. The information label of another execution of the CD is printed directly on the non-readable side of the CD prior to the attaching of the hologram label. Here a counterfeit label may be added to unauthorized copies of the CD.
The U.S. Pat. No. 5,737,298 uses a polarimeter arrangement by which the extend of the birefringence is measured in the bona fide CD in comparison to a genuine CD. Any difference between the two measurements may indicate an illegal copied CD. This effect is based on the minute geometrical differences in the depth of the "pits" with respect to the "land" (plane) of the CD due to the illegal copying process. The minute geometrical differences cause locally phase differences, which in turn has a polarizing effect to the reflected light.
Several manufacturers of CD add, to the master, a mosaic of diffractive gratings or a hologram outside of the data structure area of the CD, e.g. the mosaic a notice of copyright and a logo around the center hole.
In contrast to the bright holograms creating a variable visual impression, the U.S. Pat. No. 4,501,439 teaches the construction of an visually inconspicuous, completely transparent plastic laminate label for bank notes having an optical marks made from diffractive gratings embedded within the laminate. The information stored in the optical marks is only machine-readable while the optical marks are too faint to be visually recognizable. Examples of overlay foil construction incorporating diffractive structures are described in U.S. Pat. No. 5,104,471. Other only machine-readable diffractive gratings on telephone cards are known from the U.S. Pat. No. 4,119,361. The diffractive gratings are covered by black plastic layers which are transparent for infrared light but not for light in the visual range of the radiation spectrum.
An information carrier according to U.S. Pat. No. 5,886,798 (formerly Ser. No. 08/664,453) has optical marks which are only recognizable on a screen if illuminated with coherent light. The fine structured optical marks do not reveal their information in non-coherent light.
The U.S. Pat. No. 6,060,143 by the same inventors teaches Double-Layer Security Devices which are a combination of two aligned diffractive features and the use of the parallaxes.
Machine readable holograms on CD for reasons other than for security reasons are reported by the GB-A 2 152 695 and the U.S. Pat. No. 5,754,520. Here, music or other digital information are stored much more densely on the CD by spot-like holograms along a track rather than by the "pits and land" formation of an usual CD as pointed out in Encyclopedia of Polymer Science and Engineering, vol.11, pages 657, 675-677, John Wiley & Sons, ISBN 0-471-80943-8. Machine-readable patterns of juxtaposed fields of diffractive gratings are known from the application EP 0 718 795 A1.
The U.S. Pat. No. 4,856,857 serves as a compilation of materials which can be used for laminates with embedded transparent or reflective holograms.
The GB-B 2 082 593 teaches methods to modify the refractive index of polymer resins used in machine-readable diffractive security labels.